My question is this: Could a high power laser create a visible plasma channel through its path. I ask because laser beams are invisible in clear air, but what if it was powerful enough to ionize the air into plasma? Would there be some type of visible laser channel?

I believe they could IF the power absorbed by the channel of air is sufficiently high, as that would heat the air high enough to ionize it. I believe small particles present in the air may make this easier. But it's got to be a LOT of power in a small amount of time.

Back in the 1980's there was some research on laser-steered charged particle beams for anti-missile systems. The notion was to use high-intensity beams to ionize a channel for the beam by the means Drakkith described. I think the concept was dropped when it was realized that a high-intensity laser could be more easily constructed that was by itself sufficient to disrupt a ballistic missile. A prototype airborne anti-missile laser was built and successfully tested (see "Airborne Laser Laboratory", "Airborne Laser Testbed", "Boeing YAL-1"). This program was canceled by the Obama administration.

Could a high power laser create a visible plasma channel through its path. I ask because laser beams are invisible in clear air, but what if it was powerful enough to ionize the air into plasma? Would there be some type of visible laser channel?

Powerfull laser beam of visible wavelength is visible in the air under day light. For example 532-nm laser beam with few millimeters in diameter can be visible at power of several watts. The main reason of its visibility is scattering on dust particles. But even in dust-free air there is some scattering as well.

Concerning plasma channels there is such phenomenon as laser breakdown of air. It takes place near focus of a focusing lens. There is an examle - . Here the laser spark is very short, but its length can be made much longer using powerful laser and long-focus lens. E.g. I'd observed about 2-m long lasers sparks with aid of 50-J, 10 ns lasers pulses.

Very long plasma channels can be produced using very-high-power fs lasers pulses. At this level of power self-focusing of the beam in air is obtained making the beam to be self-collimated. http://www.dmphotonics.com/fs_filamentation.htm